LEAVES 17 
tribution it is evident that stomata are definitely related to 
air; and that where there is difference of illumination on the 
two surfaces they occur chiefly on the less illuminated sur- 
face. Stomata are not peculiar to the epidermis of leaves; 
for they are found in the epidermis of any green part, as 
young stems, fruit, etc., and even on the colored parts of 
flowers. 
(2) Mesophyll.—A cross-section of a leaf such as that of 
the lily shows the single layer of epidermal cells bounding 
the section above 
and below, pierced 
here and there by 
stomata, recognized 
by their guard-cells 
(Fig. 12). An en- 
larged view of a sec- 
tion of a single sto- 
ma may be seen in 
Fig. 20. Between 
these two epidermal 
layers is the mass 
of green tissue mak- 
ing up the body of 
the leaf, and known 
as mesophyll. This 
comprises cells con- Fie. 12.—Cross-section of a lily leaf, showing epi- 
Bod dermal layers (e) with stomata (s); mesophyll 
taining the numer- made up of palisade tissue (p) and spongy tissue 
ous small green (sp) with air-spaces (a), and containing chloro- 
plasts; and sections of veinlets (v) 
bodies (chloroplasts) 
that give color to the whole leaf. Usually the mesophyll 
cells are arranged differently in the upper and lower regions 
of the horizontal leaf. In the upper region the cells just 
beneath the epidermis are elongated at right angles to the 
surface of the leaf, and stand in close contact, forming the 
palisade tissue. In the lower region of the leaf the cells 
